Considerable effort has been directed to provide a security document difficult to counterfeit. Means were sought whereby the genuineness of the security document can be effectively discerned. In order to be acceptable, the means only should be detectable under limited circumstances, and should otherwise be undetectable. Acceptable means should be capable of encoding. When detected, such means should have a clear and unmistakable presence, should withstand wear, aging and other physical impairment of the substrate; and should be economically feasible.
The code should not be obviously visible, should not be capable of application by standard printing techniques, should be applicable to various types of substrates employed to form document of a security nature, should require high cost, highly specialized equipment for application, yet provide a document only marginally increased in cost over the non-coded product. Further, the coating of which the code is formed must be formed of a material which is non-toxic, stable, and detectable with low cost equipment.